1. Field
The invention is in the field of amplifiers for musical instruments, particularly instruments such as electric guitars and electric basses.
2. State of the Art
With electrical instruments such as electric guitars and electric basses, the characteristics of the sound produced are not as dependent upon the instrument itself as upon the instrument's pickup and amplifier. The particular pickup used determines the relative amplitude of various frequency signals sent from the instrument to the amplifier. The signal processing done by the amplifier determines the characteristics of the amplified signal, and hence its audio characteristics when the electrical signal is converted to an audio signal by a speaker.
There are currently many instrument amplifiers on the market and each produces a somewhat different instrument sound. The difference may be small between some amplifiers and large between others.
It is not unusual today for a versatile performer to have several instruments and several amplifiers and use one or another depending upon the desired type of music to be played along with its characteristic sound. For example, a performer playing rock music generally desires a hard, driving sound which is characterized by an emphasis on treble volume. On the other hand, a performer playing jazz desires a more mellow sound which is characterized by an emphasis on bass volume, but still producing sharp clear sounds. A performer playing country music will generally prefer an in-between sound.
One of the principal differences between amplifiers, and one that results in the largest difference in the sound produced by the amplifier, is the frequency response of the amplifier. Most electric guitars and electric basses use a wound magnetic type of pick-up to convert string vibrations into electrical signals. These pick-ups are nonlinear, meaning that signals of different frequency produce different amplitude or strength signals. The lower frequency vibrations produce the strongest signal, and the strength of the electrical signal decreases as the frequency increases. To make up for this difference, it is necessary that the amplifier apply what is commonly called frequency compensation. In order that all frequencies of an instrument signal are included in the audio signal produced by the speaker in substantially the strength they are produced by the instrument, it is necessary to compensate for the lower amplitude high frequency signals produced by a magnetic pick-up by adjusting the amplifier so that it amplifies the high frequency signals to a greater degree than the low frequency signals. The amount of frequency compensation, where it occurs, and what frequencies are compensated to what degree are usually designed into an amplifier and are the principal reasons for the substantial differences in the sound of the output from amplifier to amplifier.
Most amplifiers have tone controls which modify the frequency response of the amplifier to some extent upon adjustment of the controls by the user. These controls generally take the form of either high pass or low pass filters in the circuit after the initial pre-amplification of the signal from the instrument pickup. The initial pre-amplification stage is where the frequency compensation takes place. Thus, the tone controls modify the compensated signal and do not act to modify the frequency compensation of the initial pre-amplification stage. Further, the tone controls are generally tied in with the volume controls so that at high volume levels, the tone controls are almost nonfunctional.
Frequency compensation is not as important where a piezoelectric pick-up or a dynamic microphone pick-up are used. These pick-ups are generally substantially linear in their output, so require a substantially linear amplification. With these pick-ups, the tone controls, if they provide a .+-.10 db tone compensation, will usually be sufficient to give any desired frequency compensation. However, as pointed out above, while such tone controls may operate satisfactorily at lower volume levels, such tone controls may not provide the desired compensation at higher volume levels (those generally used with electric instruments). Thus, even with piezoelectric pick-ups, some frequency compensation is usually required or desired.
In addition to frequency compensation in amplifiers, it is sometimes desired to introduce changes into the signal to give it a special, modified sound. One popular modification is commonly referred to as "distortion". In present amplifiers, this is generally achieved by clipping the sign wave music signal to form a wave similar to a square wave. This produces what is commonly called "fuzz tone". The problem with this procedure is that a square wave emphasizes the odd order harmonics and cancels the even order harmonics in the signal. The cancelling of the even order harmonics produces a harsh and unpleasant sound. It has been found that the most desirable "distortion" signal is one that keeps both the even and odd harmonics in about the same proportion as in the undistored signal.
Another effect often incorporated into an amplifier is the "phase shifting" effect, which produces an ethereal sweeping, swishing sound that seems to surround the listener. This can also produce the effect of a rotating speaker or vibrato. To create this effect, the frequency of a particular sound is first summed to an original frequency and then is gradually subtracted or nulled. While the currently known circuitry for achieving this effect is generally satisfactory, room remains for improvement in the circuitry both from the standpoint of simplicity and operation.
A still further effect usually incorporated into amplifiers is the so-called "reverb" effect, which results from the combination of a signal with a similar, but delayed signal. This produces an echo type of sound. The normal delay lines used in musical instrument amplifiers substantially reduce signals above six kHz thereby reducing substantially the higher frequency signals in the combined signal. No effective way to compensate for this reduction in high frequency signals when using a reverb unit is currently available.
It would thus be desirable to have an instrument amplifier where the frequency compensation of the initial pre-amplifier could be adjusted by the user to adjust for differences in pick-ups being used and to give an adjustable desired type of sound to a particular instrument. It would also be desirable to have an amplifier where the tone control would operate over its full range regardless of the volume setting, and where the distortion circuit would not unduly cancell even harmonics of the signal. Further, it would be desirable to have an amplifier with improved "phase shifting" circuitry and high frequency compensation for use in conjunction with "reverb" circuitry.